The present invention relates generally to integrated circuits, and more particularly, to a current clamp circuit for over current protection.
Many integrated circuits (IC) include a current-source circuit that receives an input signal and provides an output signal to a load circuit based on the input signal. Typically, a metal-oxide semiconductor field effect transistor (MOSFET) is used as a current-source circuit. Fluctuations in the input signal can cause the current of the output signal to fluctuate, and sometimes the fluctuations may cause the input signal to increase beyond a threshold current level, which may result in an over-heating of the load circuit, which could damage the load circuit. Hence, a current clamp circuit may be provided to prevent an increase of the current of the output signal and thus, protect the load circuit.
A conventional current clamp circuit includes a current-source circuit, a current-sense circuit, and a feedback circuit. The current-source circuit is connected to the load circuit and includes a first MOSFET. The current-sense circuit includes a second MOSFET and an operational amplifier (op-amp) in a negative-feedback configuration. Each of the first and second MOSFETs has a drain and a gate that receive the input signal and a bias signal, respectively. A source of the first MOSFET provides the output signal to the load circuit. The source of the second MOSFET provides a sensed current signal. The op-amp is connected to the sources of the first and second MOSFETs to receive the output signal and the sensed current signal, respectively. The op-amp ensures that a voltage level of the sensed current signal is equal to a voltage level of the output signal. Hence, gate-to-source and drain-to-source voltages of the first MOSFET are approximately equal to gate-to-source and drain-to-source voltages of the second MOSFET, respectively. Since the current output from a MOSFET is based on the gate-to-source and drain-to-source voltages, the current of the sensed current signal is approximately equal to the current of the output signal.
The feedback circuit is connected to the current-source and current-sense circuits and generates the bias signal based on the sensed current signal. The bias signal controls a voltage level at the gates the first and second MOSFETS. Thus, when the current of the sensed current signal (which is equal to the current of the output signal) is equal to the threshold current, the feedback circuit reduces the voltage level of the bias signal to limit the current of the output signal.
The first and second MOSFETs may include first and second parasitic resistors having resistances that are not equal to each other, so a voltage drop across the first parasitic resistor will not be equal to a voltage drop across the second parasitic resistor. Thus, the drain-to-source voltage of the first MOSFET will not be equal to the drain-to-source voltage of the second MOSFET. Hence, the current of the sensed current signal will not be equal to the current of the output signal. Thus, the current-sense circuit does not accurately sense the current of the output signal and hence, the bias signal output by the feedback circuit may have a voltage level that fails to limit the current of the output signal below the threshold current. Further, since the resistances of the first and second parasitic resistors are small, i.e., in the range of a few milliohms, they are difficult to measure and rectify.
A known technique to ensure equal voltage drops across the first and second parasitic resistors is to connect a compensation resistor between the source of the second MOSFET and the op-amp. A voltage drop across the compensation resistor and the second parasitic resistor is approximately equal to the voltage drop across the first parasitic resistor. However, the resistance of the first and second parasitic resistors may change due to package stress and process-voltage-temperature (PVT) variations, whereas the resistance of the compensation resistor is fixed. Hence, the PVT variations and package stress account for unequal drain-to-source voltages of the first and second MOSFETs. Thus, the current-sense circuit may not accurately sense the current of the output signal despite the use of the compensation resistor.
It would be advantageous to have a current clamp circuit that accurately compensates for a parasitic resistance of a current source circuit and accurately senses a current of the output signal of a current-source circuit thereof.